GaN series compound semiconductors such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have been in the limelight as materials for blue light-emitting diodes (LEDs) and laser diodes (LDs). Further, GaN series compound semiconductors begin to be applied and developed to elements for electronic devices by utilizing the characteristics of good heat- and environment-resistance.
GaN series compound semiconductors have difficulty in bulk crystal growth, and therefore a GaN self-standing substrate which can withstand practical use is still developing. A substrate for GaN growth which has currently been widely put into practical use is sapphire, and a method for epitaxially growing GaN on a single-crystal sapphire substrate by metal organic vapor phase epitaxy (MOVPE) is generally used.
Since the sapphire substrate has a different lattice constant from GaN, a single-crystal film cannot be grown by directly growing GaN on the sapphire substrate. For this reason, a method has been developed for once growing an AlN buffer layer on the sapphire substrate at low temperatures, relieving lattice strain by this low-temperature grown buffer layer, and growing GaN thereon (Japanese patent application laid-open No. 2-81484).
GaN single-crystal epitaxial growth has been made possible by using this low-temperature grown nitride layer as the buffer layer.
However, the lattice mismatch between the substrate and the crystal cannot be obviated even by this method, and GaN obtained has a dislocation of 1019-1010 cm−2. This defect becomes an obstacle in fabricating a GaN series LD.
In recent years, such growing techniques as ELO (Appl. Phys. Lett. 71 (18) 2638 (1997)), FIELO (Japan. J. Appl. Phys. 38, L184 (1999)), pendeoepitaxy (MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999)) have been reported as methods for reducing defect density caused by the lattice constant difference between sapphire and GaN.
These growing techniques have been for preventing dislocation propagation from a base crystal by forming a patterned mask with SiO2 or the like on GaN grown on a substrate of sapphire or the like, further selectively growing a GaN crystal from window portions of the mask, and covering the mask with the laterally growing GaN.
The development of these growing techniques has allowed the dislocation density in the GaN to be remarkably reduced to the order of 107s cm−2. For instance, Japanese patent application laid-open No. 10-312971 discloses one example of this techniques.
The above-mentioned ELO and other low-dislocation GaN growing techniques all require a process for forming a patterned mask with SiO2 or the like on a substrate of sapphire or the like. This process comprises a SiO2 film depositing step by CVD or the like, a resist coating step, a photolithography step, an etching and cleaning step, etc., so that it is very complicated and time-consuming.
Since fine processing technology is also required, there is the problem that the yield (reproducibility) of the mask formation is poor. Further, since the present process involves many heat treatment steps and cleaning step, the risk of contamination and damage of the substrate by handling is high.
While the above techniques involve the complicated process as explained above, the dislocation density of the GaN crystal obtained is not necessarily a satisfactory value for LD development.
This is considered to be because strain is caused in the growing GaN by the difference between areas having the mask and no mask for selective growth, and a crystal axis is thereby inclined, as reported in Appl. Phys. Lett., Vol. 76, No. 26 (2000) 3893-3895, J. Crystal Growth 208 (2000) 804-808, etc., for example.